The present invention relates to foundry moulding compositions useful for the production of moulds or cores required for the manufacture of metal castings. More particularly, the invention relates to compositions useful for producing foundry moulds or cores which, without the need of separate blacking applications, are capable of giving castings of good surface finish.
Conventionally, in the production of metal castings, particularly in the case of castings of grey and nodular irons, aluminum and low melting point alloys such as bronze and brass from moulds formed from cold set resin bonded sand, the surface finish of the castings is improved by applying to the surfaces of the moulds and/or cores a wash known as a blacking prior to casting. Such washes commonly comprise a suspension of carbon or graphite in a liquid carrier such as water or a low boiling organic solvent, for example isopropanol. After application, the carrier is evaporated or, if a flammable liquid is used, may be ignited.
There is some dispute as to the precise mechanism of the action of such washes, and their action may involve a number of different effects. It is, however, generally believed that the solid particles contained in the washes acts in a mechanical way by filling the voids and cracks in the mould or core surface. The carbon present in the wash or produced by the action of the hot metal during the casting operation may serve as a release agent by creating a barrier between the mould wall and the solidifying metal. It has also been suggested that the wash serves to release gas to form a gas cushion between the mould walls and the molten metal. A general discussion of these effects can be found in Trans. AFS, Vol. 82, pages 169-180 (1974). However, whatever mechanism, or combination of mechanisms, is followed, such washes are found to improve the surface finish of castings made from moulds produced from many types of binder.
The need to apply blacking washes is, however, a disadvantage. Firstly, it involves a separate, often very time-consuming step, which adds additional labor cost to the production of castings. Secondly, the blacking washes are difficult to apply uniformly to the surfaces of the moulds and cores, especially in the case of complex mould and core shapes. Consequently, their efficiency will vary from mould to mould and from point to point within a mould. It is a further disadvantage when a flammable carrier solvent is used in that not only does this involve an additional material cost but it also results in the production of fumes which may consequently reduce the quality of the working environment, as well as constituting a flammability hazard requiring special storage conditions and subsequent caution during use.